This application claims priority of European Patent Application No. 99307501.9, which was filed on Sep. 22, 1999.
1. Field of the Invention
The present invention relates to power splitters in serially connected arrangements, and particularly but not exclusively to optical power splitters in a double-fibre ring architecture, e.g. for network protection.
2. Background of the Invention
In an access network deploying a common feeder link for a large number of subscribers, like the amplified passive optical network (PON) architecture such as adopted by the PRISMA project (Photonic Routing of Interactive Services for Mobile Applications), route protection for the most failure-prone parts of the network is desirable. In some situations route protection may be essential to guarantee a sufficiently high level of system availability.
In networks installed in an urban environment, cable cuts are a major cause of system outage. Assuming that the probability of a cable cut is proportional to the cable""s length, the most effective measure to improve the system""s availability is to provide route redundancy in the longest cable sections of the network. Two parallel routes may be installed in such a section, with a 1xc3x972 optical protection switch that is flipped in case of a cable cut. The probability of system failure due to cable cut without such protection is equal to pxc2x7L1 (where p is the cable cut probability per unit length, and L1 is the length of the cable), whereas it reduces to (pxc2x7L1)2 using the protection scheme.
In field-installed networks, it is common practice to provide route redundancy by a double-ring network topology, where in normal operating conditions the signals are transported clockwise from an OLT (Optical Line Termination) to the ONUs (Optical Network Units) along one ring via the links between the nodes, and vice-versa along the same paths. In case of a link failure, the signals are transported both clockwise along one ring and counter-clockwise along the other ring (from OLT to ONUs, and vice-versa along the same paths in each ring), in order to reach the otherwise disconnected nodes. Both rings may be laid out as a linear bus network, the two fibres sharing the same cable sheet, or the same duct. The protection switch is set in a position to deliver all the power in the clockwise direction in normal operation. In the event of link failure (such as a cable cut across both fibres), it is set in a power-split position. From each node, the signals are fed to subsequent power splitting and wavelength-routing stages as, for example discussed in PRISMA""s system architecture.
In such network architectures, no accurate control is provided over the power of the optical signal delivered by each node. Any attempt to control such power distribution is further complicated by the variations in the number of nodes in any particular network caused by nodes being removed (due to a connection failure for example) or added.
According to the principles of the invention, an improved technique is provided for distributing the power of a signal in an arrangement in which a power signal is delivered to a plurality of nodes arranged in a serial manner. In one illustrative embodiment, there is provided a network architecture comprising a first set of N serially connected power splitting centres connected at one end to receive a power signal and at another end to a termination point, each splitting centre having an output associated with a sub-network for delivering a portion of the power signal to that sub-network, wherein the power splitting factor in each power splitting centre is variable such that the portion of the power signal delivered to each sub-network is variable.
The power splitting factor in each power splitting centre may be such that the portion of the power signal delivered to each sub-network is identical. The power splitting factor in each power splitting centre may follow the recursive relation:             p              i        -        1              =                  1        -                              1                          1              +                                                a                                      i                    -                    1                                                  ·                                  p                  i                                                              ⁢                      xe2x80x83                    ⁢          for          ⁢                      xe2x80x83                    ⁢          i                    =      2        ,  3  ,  …  ⁢      xe2x80x83    ,  N
where pixe2x88x921 is the power splitting factor in the ixe2x88x921th power splitting centre, and aixe2x88x921 is the attenuation loss between the ith and the ixe2x88x921th power splitting centres, and pN=1 (as this is the last node in the series).
The network architecture may include a second set of N serially connected power splitting centres connected at one end to receive the power signal and at another end to a termination point, each of the second set of optical splitting centres being associated with one of the first set of optical splitting centres such that the nth power splitting centre of the first set is associated with the (Nxe2x88x92n+1)th of the second set, and such that each pair of power splitting centres have a respective output connected to a common sub-network; wherein only one of each pair of power splitting centres receives the power signal.
The power splitting factor in each power splitting centre of the second set may be such that the portion of the power signal delivered to each sub-network is identical.
The power splitting factor in each power splitting centre of the second set may follow the recursive relation:             q              i        -        1              =                  1        -                              1                          1              +                                                a                                      i                    -                    1                                                  ·                                  q                  i                                                              ⁢                      xe2x80x83                    ⁢          for          ⁢                      xe2x80x83                    ⁢          i                    =      2        ,  3  ,  …  ⁢      xe2x80x83    ,  N
where qixe2x88x921 is the power splitting factor in the ixe2x88x921th power splitting centre of the second set, and aixe2x88x921 is the attenuation loss between the ith and the ixe2x88x921th power splitting centres of the second set; qN=1.
The network may further comprise a central power splitting centre for delivering the power signal to the first and second sets of power slitting centres, the central power splitting centre having an input for receiving the power signal, a first output for delivering the power signal to the first set, and a second output for delivering the power signal to the second set, wherein the central power splitting centre has a variable power splitting factor.
In normal operation the variable power splitting factor in the central power splitting centre may be set such that the power signal is all delivered to the first output.
In the event of a failure in the serial link of the first set of power splitting centres, the variable power splitting factor in the central power splitting centre may be set such that a portion of the power signal is delivered to both the first and second outputs.
If the serial connection is lost between the kth and k+1th power splitting centres of the first set, the first k of the power splitting centres of the first set may receive a portion of the power signal, and the first Nxe2x88x92k of the power splitting centres of the second set may receive a portion of the power splitting centre. The power splitting factor in the central power splitting centre may be             q      1                      p        1            +              q        1              ,
where p1 is the power splitting factor in the first power splitting centre of the first set, and q1 is the power splitting factor in the first power splitting centre of the second set.
The power splitting factor in each active power splitting centre of the first and second sets may be set such that the power level delivered to each sub-network is the same.
Each power splitting centre may be an optical power splitting centre. The network may comprise a passive optical network.
The network may comprise a feeder network for any one of a wireless communication system, a system deploying coaxial cables, a system deploying twisted copper pair cables, etc.
The network architecture may comprise a double fibre ring architecture, the first set of power splitting centres comprising a first fibre ring and the second set of power splitting centres providing a second fibre ring.
The double ring protection architecture using a single-fibre bus topology per ring is a fibre-lean network architecture. In order to obtain a constant output power per node and thus to allow a modular ONU design, the power split factor per node is dependent on the node""s position. Deploying a power splitter with a widely tunable split factor according to the present invention will facilitate the insertion of new nodes, and hence enable scalability of the network.